Feline diarhea

Question
My cat will be one year old in May, and we've had him since he was six weeks old. We have another cat who is at least six years old who has no problems using the litter box (only one, but is large). My young cat, ever since he was neutered in December, has taken to urinating around the house on carpeted areas (he sometimes goes in the litter box, but when we're not home he sometimes goes elsewhere). Yesterday and today I found diarhea on the carpet by the door to the bathroom, where the litter box is. I think it was the little one, but am not sure. He has deficated on the carpet before, usually hidden between two doors by the bathroom, or in the bathtub. Is there anything I can do to help him with the diarhea, and can I do anything to get him to use the litter box all the time?--I actually took away their food this morning and there were no "surprises" on the carpet, but there was a little more solid but still liquidy poo in the litter box (I also kept them both in the bathroom/bedroom--normally they have free range of the apartment)  Should I just keep doing what i'm diong for the diarhea?  But is there something I can do about my kitten's peeing around on the carpets, also?

Answer
Hi Michelle!  Generally when a cat eliminates outside the litterbox, it means they are not feeling at their best. Here are a few things that come to my mind:  You mentioned that he has begun this behavior since he was neutered.  Sometimes, the stress of undergoing anesthesia can cause a slight behavioral problem, which will go away in time.  When in surgery, the animals are intubated, which can cause ulcerations along the intestinal track, which is painful, can cause fever, and can cause diarrhea.  
The number one reason for urination outside the litterbox is from a Urinary Tract Infection.
The number one reason for diarrhea would be an intestinal parasite, change of food, or eating something they shouldn't.
The only way that you will get an answer would be by taking him to your veterinarian.  All of these ailments can be cured with antibiotics and medications.  To prevent him from urinating around the house after he is better, you will need to get the urine scent out of your carpet.  Cats have very sensitive noses and once they smell their scent, they will continue to eliminate there.  Dawn Dish detergent and Nature's Miracle pet odor remover work the best.
I hope this has been helpful and I hope he feels better!
Hilary